Getting a grip The evolution of the hand—particularly the opposable thumb—was key to the success of early humans. Without a precise grip, involving forceful opposition of thumb with fingers, tool technology could not have emerged. Skinner et al. analyzed the internal bone structure of Pliocene Australopithecus hands, dated at 3.2 million years old. Internal bone structure reveals the patterns and directions of forces operating on the hand, providing clues to the kinds of activities performed. Modern human-like hand postures consistent with the habitual use of tools appeared about half a million years earlier than the first archaeological evidence of stone tools. Science , this issue p. 395